Code:
lvcreate -V 1000G -T pve/data -n backups
But.... doing -V means he didnt count the available size properly, and is over provisioning, which is fine, if you actually put that amount of data on there you will have a bomb unless you add a disk to the group, it is suggested to count your GBs/extents, and underprovision, you can always extend later when you add new disks...
on top of all this, it appears he is placing backups on the pve data volgroup, which is fine if you know what that means.... that means unless he has another storage location where VMs are running from, he is backing up to the same location as where they are running, in the case that the volgroups crashes, you have lost everything.
Ideally you setup a separate location for your backups, format it something, ie ext4, mount it on a directory, ie /mnt/backups, and add that storage location in the gui.
If you understand the consequences of this, and wanted put something on the data volgroup, check the size available in the pool using lvdisplay.
subtract and VMs you have running on it, or over provision (thats the point of Thin - right) and keep a close eye on your available space, having a VM with 200gb used, and storing 5 backups per week is recipe for disaster, the backup will fail smartly when space is low, but it could potentially get close to the limit, and if you consume additional space in your vm - kaboom.